Optical constants of vacuum evaporated thin films in the Ge1-xSe2Pbx (x = 0, 0.2, 0.4, 0.6) system were calculated from reflectance and transmittance spectra. It is found that the films exhibit non-direct gap which decrease with increasing Pb content. The variation in the refractive index and the imaginary parts of the dielectric constant with photon energy have been also reported. The relationship between the optical gap and chemical composition in chalcogenide glasses is discussed in terms of the average heat of atomization.